Late last week, the New York Times Magazine had a Q & A regarding a scientist's obligation to respond to crackpots spreading pseudoscience and antiscience attitudes. The response was basically in 3 parts. First, if you encounter it directly in your job, confront it. Second, if you have a crackpot friend/family member/acquaintance who isn't really hurting anyone, then try at least once to correct their ways, but humor them thereafter. Third, if the crackpot now tries to spread their views to broader society, then you have to stop them.
If don't have a problem with the first two parts of the answer, but the third part is problematic. First, crackpotiness is not a binary state, but rather a continuum. John Baez has posted an Crackpot Index (as have others) with the output being a number. Some crackpots are going to score higher than others, so that should figure into how to respond to your crackpot neighbor. I would suggest that the urgency to respond is not linear with the degree of crackiness, as top of the scale crackpots are so goofy that most people won't take them seriously at all. It's the crackpots in the middle, the ones that are still sane enough most of the time that have the credibility to be taken more seriously.
The sentence "Now, that responsibility changes if the misinformation starts to spill into the rest of society" creates a huge burden for scientists. While the examples given (preventing them from teaching a 2nd grade class about unicorn, etc.) are pretty simple, society isn't just limited to our local geography which is what the additional examples in the Q & A are. More than ever, crackpots are posting their ideas on the web and interacting with others with similar ideas. Since the web is clearly "the rest of society", are we now burdened with the charge to respond to every site? To try and shut the sites down?
Beside reeking of censorship, this approach would be fairly ineffective at best. Crackpots revel in the idea of their lone genius taking on the whole world, and so taking aggressive attitude against them only spurs them on. And it is very rare that logic and facts could ever change their minds. Crackpots are people who have fallen in love with an idea and continue to seek facts to support it rather than question it anew. Something that even scientists (at times) are equally culpable of.
My advice would be to confront crackpots to follow the first 2 parts of the advice columns answer, but as for crackpots in broader society, confront them as you wish, but realize that you will be unable to change their minds. Put some effort into, but don't feel obligated to respond to anyone that you encounter. The reward is too little for the effort exerted.